1. Field of the Invention
The present invention relates generally to wide-range tunable semiconductor lasers and particularly to sampled-grating distributed Bragg reflector (SGDBR) lasers.
2. Description of the Related Art
Diode lasers are being used in such applications as optical communications, sensors and computer systems. In such applications, it is very useful to employ lasers that can be easily adjusted to output frequencies across a wide wavelength range. A diode laser which can be operated at selectably variable frequencies covering a wide wavelength range, i.e. a widely tunable laser, is an invaluable tool. The number of separate channels that can be switched between by a laser source in a given wavelength range is exceedingly limited without such a laser. Accordingly, the number of individual communications paths that can exist simultaneously switched in a system employing such range-limited lasers is similarly very limited. Thus, while diode lasers have provided solutions to many problems in communications, sensors and computer system designs, they have not fulfilled their potential based on the available bandwidth afforded by light-based systems. It is important that the number of channels be increased and that they may be selectively utilized in order for optical systems to be realized for many future applications.
For a variety of applications, it is necessary to have tunable diode lasers which can be selectively configured to emit substantially one of a wide range of wavelengths. Such applications include transmission sources and local oscillators in coherent lightwave communications systems, sources for other multi-channel lightwave communication systems, and sources for use in frequency modulated sensor systems. Continuous tunability is usually needed over some range of wavelengths.
In addition, widely tunable semiconductor lasers, such as the sampled-grating distributed-Bragg-reflector (SGDBR) laser, the grating-coupled sampled-reflector (GCSR) laser, and vertical-cavity lasers with micro-mechanical moveable mirrors (VCSEL-MEMs) generally must compromise their output power in order to achieve a large tuning range. The basic function and structure of SGDBR lasers is detailed in U.S. Pat. No. 4,896,325, issued Jan. 23, 1990, to Larry A. Coldren, and entitled “MULTI-SECTION TUNABLE LASER WITH DIFFERING MULTI-ELEMENT MIRRORS”, which patent is incorporated by reference herein. Designs that can provide over 40 nm of tuning range have not been able to provide much more than a milliwatt or two of power out at the extrema of their tuning spectrum. However, current and future optical fiber communication systems as well as spectroscopic applications require output powers in excess of 10 mW over the full tuning range. The International Telecommunication Union (ITU) C-band is about 40 nm wide near 1.55 μm. There are other ITU bands as well that may be used including the L-band and the S-band.
It is desired to have a single component that can cover at least the entire C-band. Systems that are to operate at high bit rates may require more than 20 mW over the full ITU bands. Such powers are available from distributed feedback (DFB) lasers, but these can only be tuned by a couple of nanometers by adjusting their temperature. Thus, it is very desirable to have a source with both wide tuning range (>40 nm) and high power (>20 mW) without a significant increase in fabrication complexity over existing widely tunable designs.
The present invention discloses methods and devices of enhanced semiconductor laser, and particularly sampled-grating distributed Bragg reflector (SGDBR) lasers, which achieve high power over a wide tuning range and are manufactured using conventional techniques.